Unicondylar tibial knee implant

ABSTRACT

An implant providing for both short and long term stability and fixation is disclosed. The implant includes a plurality of projections extending from a bone contacting surface, and a porous material covering at least portions of the surface and projections. The orientation of the projections and the porous material provide for the stability and fixation. Methods of forming and utilizing the implant are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the filing date of U.S.Provisional Patent Application No. 61/794,339 filed Mar. 15, 2013, thedisclosure of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to an orthopedic medical deviceimplant. In particular, the present invention is related to aunicondylar knee implant system's tibial component.

Orthopedic knee implant systems have been used for many years to treatpatients with knee joints that have been damaged by trauma or disease,such as osteoarthritis, rheumatoid arthritis, and avascular neurosis. Aknee arthroplasty resects, cuts, or resurfaces the damaged sections ofthe knee and replaces them with an endoprosthetic or implant.

Most knee implant systems are tricompartmental implants and the surgicalprocedure used with tricompartmental implants is commonly known as totalknee arthroplasty. These implants are known as tricompartmental implantsbecause they are used when the knee joint is prepared to receive animplant by resurfacing or resecting the three articulating compartments,i.e., the medial and lateral femorotibial and the patellofemoralsurfaces. Regardless of the type of implant used, all arthroplastiesrequire the bone to be specifically prepared to receive a correspondingimplant by resecting, resurfacing, or deforming the bone to accept theimplant.

Unicondylar or unicompartmental knee implants have become of greatinterest in the orthopedic industry due to their less invasive natureand the maintaining of the other healthy knee compartments. Unicondylarknees resurface or resect typically the medial or lateral femorotibialarticulating surfaces thus allowing preservation of the othercompartments which may not be suffering from damage due to trauma ordisease.

Generally, the clinical outcomes for unicondylar knee implants havevaried. Studies have reported long term survival rates for unicondylarimplants to be less than that of comparable total knee implants. Oneparticular cause for such discrepancies is due to the bone cementfixation technique associated with the tibial implant. Another cause isthe limitations on longer term cement fixation integrity. And, anothercause is the non-physiological tibial bone loading patterns of arequired metal backed tibial component that is relatively stiff comparedto the surrounding bone.

The development of orthopedic implant designs has been moving towardsmeeting the requirements of high demand patients. Patients today arerequiring more from their implants and since patients are living longer,they are requiring implants to last longer. Accordingly, developmentshave been made in materials used to make orthopedic implants to improveimplant survival rates, such as highly porous metals for biological bonefixation.

Orthopedic devices are mated with host bone by either cementing them inplace using methyl methacrylate, generally termed bone cement, or byproviding a rough or porous surface on the device for bone tissue togrow into, generally termed press-fit or cementless.

The use of bone cement in attaching a prosthesis within or onto aprepared bone provides an excellent immediate fixation but has variousdisadvantages that appear over time. Physical loads are repeatedlyapplied to the implant over its life. If bone cement is used to secure aunicompartmental knee prosthesis, the bone cement may fatigue andfracture under the repeated loading. In some instances, degradation ofthe bone cement integrity may cause the device to become loose, therebynecessitating replacement. Old bone cement must be removed from the hostbone as part of the implant replacement procedure. This procedure can becomplex, time consuming and potentially destructive to healthy bonestructures surrounding the implant. Furthermore, conventional bonecement is cured after it has been dispensed into the patient's joint.Loose undetected cement fragments can remain in the joint space and,with patient mobility over time, increase the degradation rate ofarticulating implant surfaces.

Recognizing the disadvantages of cement fixation techniques, prior artdevices have been developed that utilize mechanical attachment means tojoin an implant to bone for immediate stabilization. Various implantsurface treatments intended to bond with bone biologically for long termstable attachment have proven successful. A simple technique ofmechanically securing an implant, is to affix it within the bone withscrews or other mechanical fasteners. However, due to the nature of thebone surrounding the surgical site, and other limiting factors such asartery location and the like, screws can only be applied in certainlimited regions. The use of a screw for implant fixation should beconsidered only as an option by the surgeon depending upon implantplacement and bone quality.

Primary fixation of an implant should come from a high frictioninterface with the prepared bone and in the long term with bone tissueingrowth into a porous portion of the device. Specific instruments andsurgical procedures are developed to match the implant and bonepreparation. Often the bone cuts are undersized so that the implant or aportion of the implant such as a peg or keel is “press fit” into thebone. This assures an intimate contact between bone and implant. A highfriction coating or porous portion of the implant assists with immediatebone fixation by mechanically locking the device in place. High frictionwill also resist any loading which may displace the device prior to boneingrowth and more permanent biological fixation.

Prior art has established many methods for producing a high frictionporous layer for implant designs. The use of metal beads, particles orwires which are metalurgically bonded to the implant surface is common.Plasma coating of metal surfaces with rough layers of metal particles isalso utilized. More recently, porous metals of various chemical make upand structure have been developed which mimic the design of bonetrabecular structure. These materials have been shown to have superiorbone ingrowth results and should lead to improved implant fixation.

BRIEF SUMMARY OF THE INVENTION

In accordance with a preferred embodiment, the present inventionprovides for a unicondylar tibial implant. The tibial implant includes atibial keel positioned on a surface of the tibial implant to besubmerged into prepared bone with a first projection extending along itslengthwise direction and a second projection extending along a directionperpendicular to the first projection. The first projection may beinterrupted by a void to allow clearance for another implant orinstrument. The second projection intersects the first projection. Thetibial implant can be fabricated from a metal, a polymer, abiodegradable material, a porous metal material, or combinationsthereof. The device as described could be produced through additivemanufacturing techniques such as direct metal laser sintering. Theforegoing description of the present invention is provided for thetibial implant when used on the medial condyle. However, the preferredembodiment can also be used on the lateral condyle, and when utilized insuch a manner would have some features reversed in orientation. Adescription of the medial component features of the tibial implant isprovided only for simplification.

The tibial keel is configured as an anterior-posterior projection withan intersecting keel segment that extends about a medial-lateraldirection. The tibial keel is comprised of a solid material on a boneinterfacing leading edge of the tibial keel i.e., a solid end portion,with the tibial keel having a porous material between the tibial trayand the solid end portion of the tibial keel. The tibial implant canoptionally include a bone screw to secure the tibial implant to bone.

In accordance with another preferred embodiment, the present inventionprovides for a unicondylar tibial implant having a tibial keelconfigured as an anterior-posterior projection with at its most anterioraspect being an intersecting keel in the medial-lateral direction. Thetibial keel is comprised of a solid material on a leading edge of thekeel and porous material between the tibial tray and the solid endportion of the keel, and smaller protrusions on the medial facingportion of the tibial keel at the intersection of the tibial keel andtibial tray. The tibial implant is fabricated from a metal, a polymerand/or a biodegradable material. The tibial implant can optionallyinclude a bone screw to secure the tibial implant to bone.

In accordance with yet another preferred embodiment, the presentinvention provides for a unicondylar tibial implant having a tibial keelconfigured as an anterior-posterior projection with at its most anterioraspect being an intersecting keel in the medial-lateral direction. Thetibial keel is comprised of a solid material on the leading edge of thekeel and porous material between the tibial tray and a solid end portionof the keel being implanted into an interference-fit created by anundersized preparation in the bone. The tibial implant is fabricatedfrom a metal, a polymer and/or a biodegradable material. The tibialimplant can optionally include a bone screw to secure the tibial implantto bone.

In accordance with another preferred embodiment, the present inventionprovides for a unicondylar tibial implant having a tibial keelconfigured as an anterior-posterior projection with at its most anterioraspect being an intersecting keel in the medial-lateral direction. Thetibial keel is comprised of a solid material on a leading edge of thekeel and porous material between the tibial tray and a solid end portionof the keel, and smaller protrusions on the medial facing portion of thekeel at the intersection of the tibial keel and tibial tray where theprotrusions preferentially force the tibial implant into the boneprepared about a resected mid-tibial eminence. The tibial implant isimplanted into an interference fit created by an undersized preparationin the bone. The tibial implant is fabricated from a metal, a polymerand/or a biodegradable material. The tibial implant can optionallyinclude a bone screw to secure the tibial implant to bone.

In accordance with yet another preferred embodiment, the presentinvention provides for a keel for a unicondylar tibial implant. The keelis connected to the tibial tray of the tibial implant and includessmaller protrusions on a medial facing portion of the keel at anintersection of the keel and the tibial tray where the protrusions pushthe tibial implant into the bone prepared about a resected tibialeminence. The keel is fabricated from a metal, a polymer and/or abiodegradable material. The tibial implant can optionally include a bonescrew to secure the tibial implant to bone.

In accordance with another preferred embodiment, the present inventionprovides for a unicondylar tibial implant having a tibial tray with aporous keel and protrusions extending from the keel. The tibial trayaccepts a polyethylene tibial bearing having an articulating surface forarticulating with a femoral component. The tibial bearing can be amodular polyethylene tibial bearing. The tibial implant and tibialbearing can also be formed as a monoblock component. Alternatively, thetibial tray with a porous keel can be formed out of a singularbiomaterial which is also used to form the tibial bearing. The tibialimplant can optionally include a bone screw to secure the tibial implantto bone.

In accordance with yet another preferred embodiment, the presentinvention provides for a unicondylar tibial implant having at least onesection of material that in its normal state forms at least oneuninterrupted surface of the implant that is separable from the greaterbulk of the tibial implant in a predictable shape defined by thepresence of a shear section. The shear section of material when removedexposes a passageway for at least one additional implant, such as a bonescrew. The removal of the shear section also exposes a passageway forsurgical instrumentation, for the application of osteobiologic materialsor for the application of bone cement.

In accordance with another preferred embodiment, the present inventionprovides for the ornamental design of a unicondylar tibial implant asshown and described in the figures below.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, there are shown in the drawings embodimentswhich are presently preferred. It should be understood, however, thatthe invention is not limited to the precise arrangements andinstrumentalities shown.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 illustrate a unicondylar tibial implant assembly in accordancewith a preferred embodiment of the present invention;

FIGS. 9-18 illustrate a unicondylar tibial implant of the tibial implantassembly of FIGS. 1-8;

FIGS. 19 and 20 illustrate the unicondylar tibial implant of FIGS. 9-18with a bone screw positioned within a through hole of the tibialimplant;

FIGS. 21-29 are highly magnified photographic images of a porous portionof the unicondylar tibial implant of FIGS. 9-18;

FIGS. 30-37 illustrate a unicondylar tibial implant in accordance withanother aspect of the preferred embodiment of the present invention;

FIGS. 38-40 illustrate a unicondylar tibial implant in accordance withyet another aspect of the preferred embodiment of the present invention;and

FIGS. 41-43 illustrate a unicondylar tibial implant in accordance withanother a further aspect of the preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention illustrated in the accompanying drawings. Whereverpossible, the same or like reference numbers will be used throughout thedrawings to refer to the same or like features. It should be noted thatthe drawings are in simplified form and are not drawn to precise scale.In reference to the disclosure herein, for purposes of convenience andclarity only, directional terms such as top, bottom, above, below anddiagonal, are used with respect to the accompanying drawings. Suchdirectional terms used in conjunction with the following description ofthe drawings should not be construed to limit the scope of the inventionin any manner not explicitly set forth. Additionally, the term “a,” asused in the specification, means “at least one.” The terminologyincludes the words above specifically mentioned, derivatives thereof,and words of similar import.

Partial knee implants, also known as unicondylar or unicompartmentalknee implants, replace either a medial or lateral compartment of a kneejoint by resurfacing, either by itself or in conjunction with aresurfacing of the femoral condyle and an articulating surface of aproximal tibia with an engineered implant. The preparation of the boneto accept such implants may be facilitated by instrumentation such asbone files, burrs, saws, punches, and/or computer assistedinstrumentation/navigation systems. Once the bone is prepared, theimplant may be secured to the bone by bone cement which bonds to theimplant and impregnates the bone resulting in fixation of the implant tothe bone interface.

In order to remove bone cement from the surgical procedure of implantingpartial knee implants, implants have been designed for fixation directlyto the bone. Such fixation without bone cement is known as cementlessfixation or press-fit fixation. The challenge of cementless fixation oftibial implant components is to have acceptable initial stability uponimplantation to allow patient mobility immediately or a short time aftersurgery and promote adequate biologic fixation of the implant to thebone long term. The initial stability and long term fixation arerequirements of the implant to reduce the incidence of implant looseningand reduce patient post-operative pain over time.

The present invention illustrated in FIGS. 1-43 discloses preferredembodiments of a unicondylar tibial implant assembly 5 having aunicondylar tibial implant 10 and a unicondylar tibial implant bearing12. The unicondylar tibial implant 10 has been developed primarily forcementless application and includes a unique bone interfacing tibialkeel 14 and a porous structured biomaterial interface i.e., a porousportion 16 (FIGS. 21-29). The tibial implant 10 can be constructed fromany combination of solid metal, porous metal, polymers or resorbablematerials.

For purposes of convenience only, and not by way of limitation, theforegoing description of the preferred embodiments of the unicondylartibial implant assembly 5 will be described and illustrated with respectto a unicondylar tibial implant assembly 5 for a medial tibial condyle.However, the foregoing description and features of the unicondylartibial implant assembly 5 are equally applicable to a unicondylar tibialimplant assembly for a lateral condyle, such similar features of thelateral unicondylar tibial implant assembly being substantially mirrorimages of such features of the medial unicondylar tibial implantassembly.

The tibial keel 14 is located on an undersurface of a tibial tray 18 ofthe tibial implant 10 which contacts a resected tibia bone (not shown).The tibial keel 14 is generally submerged into the bone to which thetibial implant 10 is to be implanted thereon. The tibial keel 14 canprepare its own cavity in the bone as it is inserted into the resectedtibia or it can occupy cavities within the bone previously prepared byinstrumentation or other implants. Any pre-cavities for receiving thetibial keel 14 when pre-prepared are generally smaller in size than thetibial keel 14 so as to generate compressive forces between the boneinterface and the tibial keel 14 and increase frictional forces betweenthe bone and the tibial keel 14. That is, the tibial keel 14 ispress-fitted into the bone.

Preferably, the tibial keel 14 is located on an underside of the tibialtray 18 of the tibial implant 10 and constructed out of a combination ofa solid metal substrate and a porous portion 16 on the surfaces of thetibial keel 14.

The tibial keel 14 is best shown in FIGS. 2, 4-10 and 14-20 and includesa first projection 20 which is generally planar and has a height whichcorresponds to a depth within a prepared bone to which the tibial keel14 will protrude into. The tibial keel 14 also includes a secondprojection 22 which is generally planar, has a height which correspondsto a depth within a prepared bone to which the tibial keel 14 willprotrude into and is substantially perpendicular to the first projection20. The heights of first and second projections 20, 22 of the tibialkeel 14 may be variable to accommodate access limitations whilemaximizing the fixation of the tibial implant 10 into bone. Preferably,the tibial keel 14 is positioned on an underside or inferior surface 24of the tibial tray 18 with the first projection 20 running along theanterior-posterior direction. The second projection 22 intersects thefirst projection 20 towards the anterior edge of the first projection20. Both of the first and second projections 20, 22 of the tibial keelare substantially normal to the underside of the tibial tray 18.Further, the first protrusion 20 can be configured to have a height thatvaries along its length.

Each of the first and second protrusions 20, 22 of the tibial implant 10can be configured to have one or more extensions i.e., a plurality ofextensions 26. FIGS. 2 and 5 illustrate the extensions 26 extending fromthe second protrusion 22. The extensions 26 that emanate from theprotrusions are oriented out of plane with the protrusion. That is, theextensions 26 extend outwardly from the lateral surfaces of theprotrusions. The extensions 26 are designed to create and fill cavitieswithin the bone so as to create and maximize compressive frictionalforces between the tibial keel 14 and the surrounding bone. Theextensions 26 are preferably located so that resultant forces duringinsertion of the tibial implant 10 into a resected tibia bias theposition of the tibial implant 10 in a predetermined or desireddirection. The extensions 26 are configured as substantially wedgeshaped extensions that extend along substantially the entire height ofthe keel. Further, the extensions 26 preferably tapered in the distaldirection. The plurality of extensions 26 on the second protrusion 22are spaced apart from each other and substantially circumscribes thesecond protrusion 22. Preferably, the second protrusion includes fiveextensions 26, but can include more or less than five.

The extensions 26 are preferably located around the periphery of boththe first and second protrusions 20, 22 with a higher number ofextensions 26 or higher density of extensions emanating from the secondprotrusion 22 located about the anterior region of the tibial implant 10where higher frictional forces are able to make a greater contributionto address anterior lift-off stability issues of the tibial implant 10when implanted within the bone. The number of extensions 26 is greateron the sides of the protrusion 22 that face away from a central regionof the tibial implant 10 so that bone reaction forces will push/directthe tibial implant 10 into the central region of the tibia.

The tibial implant 10 can optionally be configured with a through hole28 (FIGS. 2, 5 and 21) through which another device, instrument ormaterial e.g., a bone screw 30 (FIGS. 19 and 20) can be insertedtherethrough. The through hole 28 may pass through one or more of theprotrusions 20, 22 thereby interrupting their general shape. However,material is removed from protrusions 20, 22 around or adjacent thethrough hole 28 to provide for clearance of the device, instrument ormaterial to be inserted therethrough.

A solid edge 32 (FIGS. 2, 7 and 21) at the distal end of the tibial keel14 prevents bone from growing into the tibial keel 14 from the bottomup. The majority of the surface area of the tibial implant 10 for fixingi.e., via bone ingrowth, the tibial implant 10 to the bone occurs at theperimeter of the tibial keel 14, i.e., the lateral side surfaces of thetibial keel 14. The bone which engages and contacts the bottom of thetibial keel 14 represents a small fraction of the overall surface areaof the tibial implant 10.

That is, the tibial implant 10 is configured to prevent any boneingrowth or fixation about a distal surface of the tibial keel 14 viathe solid edge 32. Preventing bone ingrowth about the distal surface ofthe tibial keel 14 allows for easier removal of the implant, ifnecessary, since bone ingrowth on such distal surfaces of the tibialkeel 14 represents areas that are most problematic to achievingseparation of the implant from bone during revision procedures. In otherwords, as an implant is pulled out of bone, bony ingrowth into thebottom portion of the tibial keel might not separate from the greatervolume of the bone exactly at the implant interface but rather somewheredeeper within the volume of bone beneath the implant. If this occursduring implant removal, the additional bone that would otherwise beinadvertently removed would complicate the revision procedure and drivethe use of more significant revision components.

The general shape of the tibial keel 14 is designed to maximize surfacearea to volume ratio for the tibial keel 14 to enhance bone ingrowththereto while minimizing the amount of bone removal during bonepreparation. The amount of surface area available for bone ingrowth isimportant for both short and long term fixation of the implant to thebone. Short term fixation is achieved by “press-fitting” a larger bodyinto a smaller preparation. Once in place, the residual stresses fromthe compressed bone around the tibial keel 14 increase the frictionalforces against the tibial keel 14 and increase the stability of thetibial implant 10 into the prepared bone. Increasing the surface areaover which the press-fit interference is effective helps to increase thetotal frictional forces available to contribute to stability of theimplant and to distribute frictional forces over a greater effectivearea of the tibial implant 10.

Long term fixation of the tibial implant 10 is enhanced by the areas ofthe tibial implant 10 having the porous structure and surface, hereafterreferred to as ‘porous metal’ 26. As the bone remodels and grows intothe porous metal 26, the frictional retention forces will be replacedand/or supplemented with bone ingrowth. The degree of this fixation viabone ingrowth is, in part, a function of the amount and distribution ofthe porous metal surface area available for ingrowth. The largedistributed tibial keel surface area thereby provides a structure forincreased stability via a larger area of bone ingrowth.

The tibial keel 14 also includes a plurality of fins 34 which extendbeyond the nominal volume of the tibial keel 14. The fins 34 enter bonethat has not been prepared to receive the fins 34. Instead, the fins 34prepare their own receiving volume within the bone as they are insertedinto the bone, i.e., the fins 34 create their own preparation into thebone. In other words, the fins 34 are inserted into bone without theneed to prepare the bone to receive such fins 34. The fins 34 are sizedto maximize their surface area, minimize their volume and are shaped toease entry into the bone. As shown in FIGS. 2 and 7, the fins 34 arepreferably configured as shown and are substantially wedge shaped orshaped as a dual inclined plane structure. Further, the fins 34 aretapered as they extend from a proximal end of the tibial keel 14distally. The fins 34 are also preferably configured to extend anoverall length about half way the overall height of the tibial keel 14.

Preferably, the through hole 28 is shaped and sized for the passage ofthe bone screw 30 (FIGS. 19 and 20) through a superior aspect of thetibial implant 10 into the bone beneath the underside or inferiorsurface of the tibial tray 18. The bone screw 30 can be angulated toachieve a desired direction by the user. Further, with material fromadjacent protrusion 20 removed, the protrusion 20 does not interferewith the passage of the bone screw 30 through the through hole 28. Suchbone screws 30 are readily known in the art and a detailed descriptionof their structure and operation is not necessary for a completeunderstanding of the present invention.

The tibial implant 10 may employ the use of a knockout plug 36 formedwithin the through hole 28 and out of a material that is metallurgicallycontinuous with the greater bulk of the tibial implant 10. The knockoutplug 36 is configured to be removed from the remainder of the tibialimplant 10 via a boundary shear section 38 around the plug 36. The plug36 may be machined into the tibial tray 18 or built in final formthrough an additive manufacturing process such as by direct metal lasersintering.

Preferably, the through hole 28, designed for the passage of the bonescrew 30 therethrough, is obstructed by the knockout plug 36 so that thesuperior surface 40 of the tibial tray 18 facing the bearing component12, which can be assembled thereto, is fully continuous without any paththrough which debris or material could pass through the tibial tray 18to the bone engaging underside of the tibial implant 10.

In sum, the tibial tray 18 has a through hole 28 into which a screw 30can be placed to further stabilize the tibial implant 10 to the preparedbone upon implantation. This is especially advantageous for initialimplant stability and when placing the tibial implant into bone ofquestionable density where the user/surgeon is not confident the boneitself is stable enough to support adequate short term stability.

The through hole 28 can be covered during the manufacturing process ofthe tibial implant 10 with the knockout plug or shear plug 36. Theknockout plug 36 has a weak cross section which will yield to anappropriate level of force. When the knockout plug 36 is in place, thereexists an uninterrupted tibial tray surface between the poly (i.e.,bearing component 12) and the bone interface. In the event of backsidewear of the bearing component 12, wear particles are less likely tomigrate out of the tibial tray 18 than if an already present throughhole were in place. The knockout plug 36 can optionally include athreaded stud 42 (FIG. 12), which mates to instrumentation to facilitateremoval of the knockout plug 36.

The porous metal 16 is formed from a porous structured biomaterial, andincludes a plurality of struts 44 (FIGS. 21-29) having varying lengthsand cross sections. At least one strut of the porous metal 16 has an endconnected to one or more other struts at node points 46 (FIG. 29)thereby forming the porous geometry of the porous metal 16. The porousmetal 16 also includes boundary struts 48 (FIGS. 26, 27 and 28) that areconfigured to extend beyond a nominal boundary of the porous metal 16.That is, the porous metal 16 has boundary struts 48 that extend awayfrom the surface of the porous metal 16 in a finger-like or hairfollicle-like fashion. The extending boundary struts 48 impart aroughness to the surface, the degree of which is dependent upon thenumber and length of boundary struts 48 present. The average or maindirection of the boundary struts 48 also impart a surface roughness thatvaries dependent upon which direction the device is driven forimplantation.

Preferably, the tibial keel 14 is formed from a metal substrate and alayer of porous metal 16 adjacent the substrate. The porous metal 16 onthe tibial keel 14 includes extending boundary struts 48 withunconnected ends pointing or extending towards the bottom or inferiorsurface of the tibial tray 18. Under similar loading conditions, slidingover the angled struts toward the bottom surface of the tibial tray 18will experience less frictional forces than bone sliding away from thebottom face of the tibial tray 18. Preferably, the boundary struts 48are angled about +/−10 degrees from normal to a surface of the substrateto which the porous metal 16 is applied to.

Another element of the present invention is that the boundary struts 48are oriented in a predetermined direction such that they push or aredirected towards the bone interface surface. While the surface of theporous metal 16 may exhibit characteristics of a rougher surface, theboundary struts 48 of the porous metal 16 implanted into a boneinterface embed themselves into the bone and provide a mechanicalinterlock to the surrounding bone. This is especially advantageousduring initial implantation for initial fixation purposes. In theaggregate, the plurality of boundary struts 48 significantly improvesthe overall stability of the tibial implant 10 upon initialimplantation.

Preferably, the bottom surface of the tibial tray 18 has extendingboundary struts 48′ (FIGS. 26 and 27) in a direction substantiallynormal to the bottom surface of the tibial tray 18. As the tibialimplant 10 is definitively seated against the bone interface surface,the boundary struts 48′ pierce the surface of the prepared bone toincrease stability of the tibial implant 10 to the bone.

The tibial implant 10 has the porous metal 16 on all surfaces that makecontact with bone. The surface of the porous metal 16 is tailored foreach specific region of the tibial implant 10 to have specific surfaceroughness and thereby specific amounts of friction when engaged withbone. That is, the tibial implant 10 is configured to have a porousmetal 16 with boundary struts 48 at predetermined angles dependent uponthe location of the porous metal 16 on the tibial implant 10.

In sum, the surfaces of the porous metal 16 have extending boundarystruts 48 which serve to modify the surface roughness of the tibialimplant 10. The size and average direction of the extending boundarystruts 48 impart different frictional coefficients depending upon thedirection the boundary struts 48 extend. The boundary struts 48 can alsobe directed in a direction largely normal to the surface from which theyextend from. This can have an additive anchoring effect which enhancesstability of the tibial implant 10 to the bone.

Referring to FIGS. 30-37, in accordance with another preferredembodiment, the present invention provides for a tibial implant 10′. Thetibial implant 10′ is similarly configured as tibial implant 10,excepted are follows. The tibial implant 10 includes a first protrusion20′ segmented by a void and a second protrusion 22′. The secondprotrusion 22′ is similarly configured as the second protrusion 22discussed above, but is spaced from the first protrusion 20′. The secondprotrusion 22′ has a height equivalent to the height of the firstprotrusion 20′ adjacent the second protrusion 22′. As best shown inFIGS. 34-36, the height of the first protrusion 20′ slopes towards theposterior end of the tibial implant 10′ such that the height of thefirst protrusion decreases as it extends from the anterior end towardsthe posterior end.

Referring to FIGS. 38-40, the tibial implant 10′ can alternativelyinclude a third protrusion 23′. The third protrusion 23′, like thesecond protrusion 22′, is slightly spaced apart from the firstprotrusion 20′. Preferably, the third protrusion 23′ is positioned moretowards the rear or posterior to the first protrusion and has a heightsimilar to the height of the posterior end of the first protrusion 20′to which it is adjacent to. The height of the third protrusion 23′ isnot equal to the height of the second protrusion 22′ or the height ofthe first protrusion adjacent the anterior end of the first protrusion20′. The third protrusion 23′ is also configured not to intersect thefirst protrusion 20′.

Referring to FIGS. 41-43, the third protrusion 23′ can alsoalternatively be positioned toward or about a middle section of thefirst protrusion 20′ and spaced apart from the first protrusion 20′.When positioned about the middle section of the first protrusion 20′,the third protrusion 23′ has a height substantially the same as the areaof the first protrusion 20′ that it is adjacent to.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. For example, additional components can beadded to the tibial implant assembly. It is to be understood, therefore,that this invention is not limited to the particular embodimentsdisclosed, but it is intended to cover modifications within the spiritand scope of the present invention as described above.

FIGS. 36 and 37 depict yet another embodiment tibial implant 410. Unlikethe above-discussed implants, implant 410 includes a keel 414 that is ofa unitary design. A hole or aperture 428 is situated offset with respectto the keel 414. This allows for the keel 414 to have a unitaryconstruction (i.e., it is not broken up by the hole 428 as in the abovedesigns). Like the foregoing embodiments, keel 414 includes twoprojections 420, 422, with the projection 420 including fins 434 and theprojection 422 including extensions 426. Of course, as in the aboveembodiments, either projection could include either or both of theextensions 426 or fins 434, and such structures can be of any shape andor size with respect to the projections. Moreover, keel 414 includes arounded cut out 421, which allows for a screw 430 to angulate withrespect to implant 410. In other words, the cut out 421 providesclearance for the screw 430 to move with respect to the plate indirections towards the keel 414.

It is also to be understood that the disclosure set forth hereinincludes all possible combinations of the particular features described.For example, where a particular feature is disclosed in the context of aparticular aspect, arrangement, configuration, or embodiment, or aparticular claim, that feature can also be used, to the extent possible,in combination with and/or in the context of other particular aspects,arrangements, configurations, and embodiments of the invention, and inthe invention generally.

Furthermore, although the invention herein has been described withreference to particular embodiments, it is to be understood that theseembodiments are merely illustrative of the principles and applicationsof the present invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. An orthopedic implant for replacing a portion of a bone comprising: abone contacting surface; and a keel extending from the bone contactingsurface, the keel including a first projection with a first longitudinalaxis and a second projection with a second longitudinal axis, whereinthe first and second longitudinal axes are oriented orthogonally to eachother.
 2. The orthopedic implant of claim 1, further comprising a holeconfigured to accept a bone screw at a plurality of different angles. 3.The orthopedic implant of claim 2, wherein the first and secondprojections are separated from each other by the hole.
 4. The orthopedicimplant of claim 2, wherein the hole includes a plug removable upon theapplication of a force.
 5. The orthopedic implant of claim 1, furthercomprising at least one fin associated with the first projection andextending oblique to the first longitudinal axis.
 6. The orthopedicimplant of claim 5, wherein the fin is shaped to engage the bone.
 7. Theorthopedic implant of claim 6, wherein the fin is configured to enterinto an unprepared portion of the bone.
 8. The orthopedic implant ofclaim 1, further comprising at least one extension associated with thesecond projection and extending oblique to the second longitudinal axis.9. The orthopedic implant of claim 8, wherein the at least one extensionis shaped to engage the bone.
 10. The orthopedic implant of claim 9,wherein the at least one extension frictionally engages the bone. 11.The orthopedic implant of claim 1, further comprising a porous portionadapted to allow for the bone to grow therein.
 12. The orthopedicimplant of claim 11, wherein the porous portion covers at least aportion of the bone contacting surface and at least a portion of thekeel.
 13. The orthopedic implant of claim 12, further comprising a solidportion at a distal end of the keel.
 14. The orthopedic implant of claim11, wherein the porous portion defines a first porous surface and atleast one boundary strut extending from the surface in a firstdirection.
 15. The orthopedic implant of claim 14, wherein the boundarystrut extends from 0 to 10 degrees from normal the first porous surface.16. The orthopedic implant of claim 1, further comprising a thirdprojection.
 17. The orthopedic implant of claim 1, further comprising abearing component attachable to the implant.
 18. The orthopedic implantof claim 1, wherein the implant is a unicondylar tibial baseplate.
 19. Akit comprising the implant of claim 1 and at least one other implant.20. A tibial baseplate comprising: a bone contacting surface havinganterior, posterior, medial and lateral sides; a first projectionextending from the bone contacting surface and having a first lengthextending in a first direction between the anterior and posterior ends;a second projection extending from the bone contacting surface andhaving a second length extending in a second direction between themedial and lateral sides; an aperture for receiving a bone screw; and aporous material for promoting bone ingrowth, the porous material atleast partially covering the bone contacting surface, the firstprojection and the second projection.
 21. The tibial baseplate of claim20, further comprising a third projection.
 22. The tibial baseplate ofclaim 20, wherein the porous material defines a plurality of boundarystruts extending from the bone contacting surface in a first direction.23. The tibial baseplate of claim 22, wherein the boundary strut extendsfrom 0 to 10 degrees from normal to the bone contacting surface.
 24. Thetibial baseplate of claim 20, wherein the first and second projectionsare separated from each other by the aperture.
 25. The tibial baseplateof claim 20, wherein the aperture is configured to accept a bone screwat a plurality of different angles.
 26. The tibial baseplate of claim20, wherein the aperture includes a plug removable upon the applicationof a force.
 27. The tibial baseplate of claim 20, further comprising atleast one fin or extension associated with at least one of the first andsecond projections.
 28. The tibial baseplate of claim 27, wherein thefin is configured to enter into an unprepared portion of the bone andthe extension frictionally engages the bone.
 29. The tibial baseplate ofclaim 20, further comprising a solid portion at distal ends of the firstand second projections.
 30. A tibial baseplate comprising: a bonecontacting surface having anterior, posterior, medial and lateral sides;a first projection extending from the bone contacting surface and havinga first length extending in a first direction between the anterior andposterior ends; a second projection extending from the bone contactingsurface and having a second length extending in a second directionbetween the medial and lateral sides; an aperture for receiving a bonescrew; a plug at least partially covering the aperture, the plug beingremovable upon the application of a force; and a porous material forpromoting bone ingrowth, the porous material at least partially coveringthe bone contacting surface, the first projection and the secondprojection, wherein the porous material defines a plurality of boundarystruts extending from the bone contacting surface from 0 to 10 degreesfrom normal to the bone contacting surface.